1. Field of the Invention
The arthroscopic diagnostic mechanical probe of the present invention is a computer-controlled indentation test device for articular cartilage. It provides clinicians with quantitative mechanical diagnostic data for cartilage.
Articular cartilage is an avascular soft tissue that covers the articulating bony ends of joints. During joint motion, cartilage acts as a lubricating mechanism in the articulating joints and protects the underlying bony structure by minimizing peak contact force at the joint. The mechanically superior capacity of articular cartilage, as a lubrication bearing as well as a viscoelastic shock absorber, is attributable to the structural integrity of its molecular constituents and their interactions with synovial fluid.
Once damaged, however, articular cartilage has limited or no ability to heal and often degenerates, leading to a degenerative joint disease call osteoarthritis [xe2x80x9cOAxe2x80x9d]. The most evident characteristics of OA are the loss of cartilage itself, from the joint surface, and the formation of osteophytes around the articular margin of the bony structure of the joint. Such degenerative changes usually are initiated by mechanical damage of the cartilage matrix, primarily manifested by a rupture of the dense collagen meshwork, a decrease of the proteoglycan [xe2x80x9cPGxe2x80x9d] content of the tissue, and an increase of the interstitial fluid content. As a result, the articular cartilage becomes softer and loses its normal mechanical properties, resulting in an abnormal stress/strain field in the extracellular matrix [xe2x80x9cECMxe2x80x9d]. The altered mechanical properties of cartilage creates an abnormal mechanical environment around the chondrocytes. In turn, this triggers an aberrant metabolic behavior of chondrocytes, producing a variety of matrix-destructive cycles, consisting of mechanical wear and tear of the cartilage matrix and the catabolic activities of chondrocytes of OA.
A change in the mechanical properties of articular cartilage can be considered the first detectable sign of cartilage degeneration. Some animal studies have shown that changes in the mechanical properties of articular cartilage can be used as a more sensitive indicator of early degeneration than the histological grading system. An efficient way, therefore, to prevent such osteoarthritic degeneration of cartilage entails early detection of mechanical changes in articular cartilage. Early detection will provide an opportunity to treat the patient at an early stage prior to occurrence of the aforementioned irreversible tissue damage.
2. Description of the Related Art
At present, there are two main diagnostic classification systems for osteoarthritic cartilage. The Kellgren and Lawrence grading system, Kellgren, J. H. and Lawrence, J. S., radialogical Assessment of Osteoarthrosis, 16 Ann. Rheum. Dis. 494-502 (1957), is based solely on radiographic grading, while the American College of Rheumatology criteria involves a mixture of clinical and radiographic features of joints. Both systems have been criticized for a lack of sensitivity, objectivity, and reproducibility. The absence of an accurate and objective diagnostic tool for articular cartilage may engender in inconsistent epidemiological conclusions, inconsistent identification of risk factors, and slowed development of primary preventive strategies for OA.
Dahefsky, U.S. Pat. No. 4,364,399, designed a diagnostic instrument to measure the deformation resistance of tissue, particularly the articular surface of the patella. The Dahefsky measurement surface utilized a special elastomeric indenter to measure the deformation resistance force of the articular cartilage. The measured reaction force, therefore, is not a pure function of cartilage stiffness, but a combined function of the material properties of the articular cartilage and the elastomeric indenter.
More recently, Lyyra T., et al., Indentation Instrument for the Measurement of Cartilage Stiffness Under Arthroscopic Control, 17 Med. Eng. Phys. 395-99 (1995), developed an arthroscopic indentation instrument (Artscan 1000) to evaluate the elastic stiffness of cartilage. Using the Lyyra instrument, the cartilage surface is indented by a given displacement and the indenter force is measured and then used to represent the tissue stiffness. The instrument, however, is unable to measure the cartilage thickness. Without the thickness data of articular cartilage, the true mechanical properties of the tissue cannot be obtained. Their instrument is also unable to measure the viscoelastic characteristics of articular cartilage. Viscoelasticity is the primary mechanism responsible for impact energy dissipation. The viscoelastic property, therefore, is critically important for assessing the mechanical condition of articular cartilage and is a sensitive indicator of degenerative alterations in the tissue structure. Moreover, the Lyyra system is very susceptible to human error because the indenter is manually pushed to the cartilage surface via a handheld instrument.
Athanasiou, U.S. Pat. Nos. 5,433,215 and 5,503,162, proposed an improved arthroscopic indentation probe capable of measuring the viscoelastic properties of articular cartilage. They also incorporated a tissue measurement technique using a penetrating probe method in their arthroscopic instrument. However, the penetrating needle probe can cause permanent structural damage to the articular cartilage.
All of these devices fail in a commonly important respect. None allow for a nondestructive accurate measurement of cartilage thickness during the arthroscopic mechanical probing procedure in situ. This, in turn, hampers the effectiveness of the probing devices because an accurate measurement of the thickness is necessary to quantitatively determine the tissue""s inherent mechanical properties. In fact, several studies have reported the feasibility of a high frequency ultrasonic transducer to measure the thickness of articular cartilage. It was found, however, that the accuracy of the ultrasonic thickness measurement of cartilage was limited by the fact that that the ultrasound speed in the cartilage could not be measured in situ and had to be assumed a priori. Jurvelin, J. S., et al., Comparison of Optical, Needle Probe and Ultrasonic Techniques for the Measurement of Articular Cartilage Thickness, 3 Biomechanics 231-35 (1995). An inaccurate estimation of cartilage thickness will result in erroneous data about the mechanical properties of articular cartilage.
An object of the invention to provide a probe which can be used to monitor cartilage condition before irreversible degenerative changes develop in articular cartilage, as well as to monitor cartilage condition after damaged cartilage has been surgically repaired and undergone physical therapy.
Another object of the invention is to provide a method and apparatus for calibrating the speed of the ultrasound in situ.
Yet another object of the invention is to provide a method and an apparatus to monitor and measure the viscoelastic properties of articular cartilage.
A further object of the invention is to provide a method and an apparatus to monitor articular cartilage while preventing damage to the cartilage.
Yet a further object of the invention is to provide a method and an apparatus which minimizes human error.
To achieve these objects, the invention provides for a method for examining articular cartilage, comprising:
(a) contacting a surface of articular cartilage with an ultrasonic transducer;
(b) applying a predetermined displacement, ds, to said surface; and
(c) determining a thickness of said articular cartilage as a function of the true ultrasound speed thereof, vs, which is defined by the equation:
vs=2ds/(t1xe2x88x92t2) 
wherein t1 is the echo time before applying said predetermined distance and t2 is the echo time after applying said predetermined distance.
The invention then allows the thickness, h, of the tissue to be calculated using the following function, although there are others:
h=vs*t1/2 
The invention also encompasses a diagnostic probe for articular cartilage, comprising (A) a probe handle and (B) a probe shaft, fixed to said handle; that comprises a housing cylinder having a proximal end and a distal end, wherein said probe shaft further comprises, from distal to proximal, (i) an ultrasonic transducer attached at the distal end of said housing cylinder; (ii) a plurality of strain-gauges attached to said housing cylinder; and (iii) a linear displacement actuator which is displaceably mounted to said housing cylinder.